It is desirable to perform a contact exposure step (variously known as "contact work") using relatively low-speed light-sensitive materials in a bright room during a process for making printing plates. Thus, a photographic light-sensitive material using a silver halide as the light-sensitive element has recently been developed, which is capable of being handled in circumstances which will be referred to hereafter as a "bright room". The terminology "bright room" as used herein means an environment in which the light-sensitive material can be handled for a long period of time under safe light not having a wavelength in the ultraviolet portion but consisting substantially of a wavelength of 400 nm or longer. This is attended by exposing a light-sensitive material having an extremely low sensitivity (e.g., about 1/10.sup.4 to 1.times.10.sup.5 which is the sensitivity of a conventional silver halide photographic material) with respect to visible light to a light source containing a large amount of ultraviolet rays under a safe light which substantially emits no ultraviolet rays.
On the other hand, in so-called contact work, not only is simple single sheet contact work performed (i.e., a negative image/positive image conversion occurs by contact-exposing a light-sensitive material using one image-exposed and developed photographic film as an original, followed by development), but such also includes high-image conversion work for forming so-called "super-imposed letter image". "Super-imposed leter image" means uninked portion(s) such as letters, marks, etc., existing in dot image portions and solid black portions of a printed paper.
A method of forming super-imposed letter image is described hereafter in more practical terms. That is, as shown in FIG. 1, an original is prepared by superposing (a) an assembly comprising a transparent or translucent base 1 (usually a polyethylene terephthalate film having a thickness of several hundred .mu.m) and a developed photographic film 2 (line image original) having so-called positive line images such as letters, marks, etc., adhered to the base 1 on (b) an assembly comprising a base 3 (similar in construction to base 1 described above) and a developed photographic film 4 (dot image original) having dot images. An original composed of assemblies (a) and (b) is then superposed on a photographic light-sensitive material 5 suitable for contact work so that the dot image portion is brought into contact with the emulsion layer of the light-sensitive material. The light-sensitive material can then be exposed to light and developed to form transparent line image portions in dot images.
An important point with respect to the above-noted exposure/development steps forming dot images is that a negative image/positive image conversion must be performed on the dot images and the line images in accordance with the dot area and the line width, respectively. For example, a dot image having a black area of 50% must be converted into a dot image having a white (transparent) area of 50%, while a line image having a black line width of 50 .mu.m must be converted into a line image having a white (transparent) line width of 50 .mu.m. However, as is clear from the Figure, while dot image is exposed on a light-sensitive material for contact work in a direct contact state with the silver halide emulsion layer of the light-sensitive material, a line image is exposed on a light-sensitive material for contact work through the dot-image original 4 (usually having a thickness of about 100 .mu.m) and the base 3 (usually having a thickness of about 100 .mu.m) for attaching thereto the dot image original. More specifically, the line image is exposed on the light-sensitive material for contact work as a dimmed or indistinct image through a transparent or translucent spacer having several hundred .mu.m thickness.
Accordingly, when an ordinary exposure amount of light (i.e., the amount of exposure required for complete converion of negative images to positive images in the dot area) is applied to the light-sensitive material in the aforesaid state, the white (transparent) line width of the converted line images is thinned by the influence of the dimmed exposure. On the other hand, if the exposure amount is reduced in order to decrease these adverse influences resulting from the dimmed exposure and thus completely convert the line width of the line images from negative to positive, the resulting overall area of the dot images is reduced due to this insufficient exposure.
Furthermore, attempts to reduce these adverse effects resulting from the dimmed exposure with a view toward improving the super-imposed letter image quality usually encounter the problem that traces of adhesive tapes used for fixing the line image original 2 and the dot image original 4 to each base may appear, and pinholes are also liable to appear on the resulting converted images.
Also, when a light-sensitive material used for contact work in a bright room is exposed to ultraviolet rays, pinholes (caused by the influences of dust, etc.) are liable to appear on the images formed as compared with a conventional light-sensitive material used for contact work which is handled in a dark room.
Solutions for this problem of the deterioration of the super-imposed letter image quality due to the presence of traces of adhesive tapes and the formation of pinholes in the resulting converted images are not yet known, probably due in part to the fact that most improvements in this field have so far been directed toward improving the fundamental qualitative properties (e.g., improving sensitivity, dot image quality and increasing gradation in a photographic characteristic curve) of conventional light-sensitive materials used for making printing plates.